You're right, there isn't much dialogue on this, which is a fault with the script. That doesn't change the fact that all of the quotes still support my theory, so posting them didn't help your case.
About the fleet are they having only a fleet or more than one? is the film when refering to the fleet talking about all Romulan starships or not? Picard talks about Praetor's fleet...is a "pretorian guard" like fleet, guarding Romulus surroundings and the Praetor himself, which is my personal opinion since Romulans inspire in ancient Rome, or i'm wrong and isn't? That could explain the apparition of the Valdor class after and not during the Dominion War, as part of an special force. Anyway the technologic evolution is perfectly plausible for me too.
If Shinzon is Praetor then the fleet is the whole Romulan fleet (with the possible exception of the Tal'Shiar, though with their loss in "The Die is Cast" it's unlikely they'd have the political capital to rebuild their fleet), as the Praetor is the head of the Romulan Senate and the commander in chief of the armed forces. So this is very much the Romulan flee they're talking about, with Romulan ships and Romulan crews.
About the fleet in the dark...I could argue that these words don't mean "dark"...as missguiding information but physically being at the dark side of Remus for example (someone could transcribe and post the part I typed as unclear), or being "dark" about what will happen and what to do from now on.
That's a nice though, but the context is clear, the fleet is being mislead in some fashion as to the change in government, and their support of the new government is contingent on the new Praetor following up on his promise of crippling the Federation. There's never been any other reference to "dark" as a side, unlike Star Wars there isn't a "light" and "dark" alignment to the universe, so whilst it may partially explain the dialogue, it still doesn't fit with Star Trek in general.
About the slavement of Remans we can't tell if ALL Remans live in the depht of the mines with the damned or ONLY those who are slaves. The film says UNDESIRED CASTE AT HIERARCHY, the relation Romulan-Reman is slavement or instead apartheid? which option you preffer is personal opinion as it's mine. If we chose slavement that doesn't means to live with a chain on the ankle... as in the ancient Rome may exist grades of slavement and grades of citizenship.
Except the only way for a slave in Ancient Rome to regain the citizenship was if they were originally a citizen and were taken as a slave and later freed, that doesn't apply to the Remans, they're a different species, and given how xenophobic the Romulans can be it'd hardly be a surprise if they never afforded Remans equal status regardless of personal merit. Even if we accept that not all Remans are slave laborers, they're still a subservient race, and the Romulans would be absolutely crazy to not keep an eye on them.
As before, Shinzon's comments are completely suspect, as we saw at the start of the film that he did not march on Romulus with an army, there was no fighting (even Federation intelligence would have found out about a Reman uprising), just a swift political assassination and the installation of a new government, headed by Shinzon. Keep in mind that he indicated that he had nothing but hope for peace with the Federation, and that his next act was to kidnap Picard to perform a lethal medical procedure so he could live long enough to attack the Federation with his weapon of mass destruction. Given all that, how can we possibly say that Shinzon is a reliable source of information. Also remember that he was telling this to Picard, the man he was going to murder not a few hours later, and he knew it at the time.
And above all else, you still haven't addressed the critical flaw in your theory, that for the Scimitar to be Reman there would need to be a massive intelligence failure by the Romulans. A huge ship containing an experimental weapon, armed to the teeth, was built without anyone Romulans noticing? I don't think so.